Various methods for incorporating biocides into resin compositions have been disclosed in the prior art. By way of illustration, U.S. Pat. No. 4,086,297 discloses a process for forming a solid thermoplastic composition containing a microbiocide utilizing very high levels of the microbiocide and two thermoplastic resins in conjunction with melt blending processing.
As another illustration, U.S. Pat. No. 4,663,359 discloses a process for preparing a microbiocide concentrate which is useful in plastisol systems. The process comprises mixing a porous thermoplastic resin powder with a high concentration of microbiocide at an elevated temperature sufficient to melt the biocide and open the pores of the resin, and incorporating the melted biocide into the pores of the porous resin, optionally in the presence of a carrier. The resulting product is provided as a dry, free-flowing powder containing the microbiocide in a high concentration at least about 20 times greater than the normal upper usage concentration for the microbiocide.
Unfortunately, the products produced in accordance with the above-mentioned '297 and '359 patents are solids which are frequently more difficult to process into a finished product than might be desired. Biocide dispersions (i.e., solid particles dispersed in a liquid) would avoid such solids handling problems in subsequent processing steps. Heretofore, however, suitable methods for incorporating insoluble or difficult-to-solubilize biocides into plastics to form dispersions has represented a challenge to the plastics manufacturing community. The solids in such dispersions tend to settle out over time, thus causing a non-uniform distribution of the additive in the dispersion. In addition, certain dispersions, most notably plastisols, have the tendency to increase in viscosity with increasing temperature, thus posing a risk that the dispersion will "set-up" or solidify during storage and/or handling prior to use.
One more successful effort to provide a biocide-containing dispersion is disclosed in U.S. Pat. No. 5,639,803. Briefly, this patent discloses a multi-step process comprising (a) forming an aqueous concentrate of biocide, carrier and water, (b) heating the concentrate to remove the water and (c) adding additional carrier and a heat swellable polymer to form a storage-stable dispersion of the biocide in the polymer. Example 1 of this patent application teaches heating in step (b) above to a temperature of 91.degree. C. and driving off the water. Unfortunately, it has now been found by the present inventors that this heating step causes undesirable discoloration of the concentrate, and this discoloration remains present in the final dispersion.
In view of the above, new methods for incorporating insoluble or difficult-to-solubilize biocides into plastics resins that avoid the settling, discoloration and/or viscosity increase problems of the prior art compositions would be highly desired by the plastics manufacturing community.